The present invention relates to a process for annealing steel strip. More particularly it relates to a process in which steel strip is annealed by dipping it in a molten glass bath of a low viscosity for a short period of time, advantageously in a continuous manner. Since the process in accordance with the invention makes it possible substantially to prevent the surfaces of the steel strip from being oxidized during annealing, subsequent steps such as pickling and polishing which would otherwise be necessary are not required or at least a burden of such steps may be greatly reduced, and therefore, the process in accordance with the invention is very advantageous from the view point of saving energy and resources.
The objects in annealing steel strip are to remove strains produced by working and to complete recrystallization of steel thereby to obtain a product having desired properties. It is well known in the art that when steel strip is rapidly heated to a temperature required for recrystallization of the grain the desired properties may be achieved.
A continuous process for annealing a steel strip has heretofore been generally practiced using a catenary furnace in which a combustion gas, obtained by burning a fluid fuel such as fuel oils, light oils and propane, is utilized as a heat source. With such a process, however, it is difficult, even in cases where a rapid heating technique is utilized, further to reduce the heating time and thereby increase productivity. This is because, whereas the thermal conductivity of a medium for heating steel strip greatly affects the heating rate of the strip, the combustion gas has a thermal conductivity as low as about 7.6.times.10.sup.-4 J/cm.S.K at 1000.degree. C. By setting the temperature in the furnace high, the heating time may be reduced to some extent, but such an approach requires more energy and expensive furnace materials. Moreover, when steel strip is annealed with a combustion gas, oxide scales are formed on the surfaces of the strip, and in consequence, subsequent steps of pickling, shot blast and polishing are required for the removal of the oxide scales. It should be noted that existing installations for treating waste liquors from the pickling step need to be large-sized in order to cope with environmental pollution regulations.
For annealing steel strip without oxidation, various processes have been proposed in which the steel strip is annealed in an inert or reducing gaseous atmosphere. Such processes, however, still attain only limited productivity, because of the limited heating rate attainable. Moreover, they are disadvantageous in that the gaseous atmosphere used is expensive in itself; and equipment for storing the gaseous atmosphere and means for completely sealing the atmosphere in the furnace are required, all leading to an increase of the manufacturing cost of the product.
Japanese Patent Publication No. 55-51496, published on Dec. 24, 1980, proposes an improvement to a known method for enhancing corrosion resistance of an article of austenitic stainless steel in which the article is annealed in an atmosphere of decomposed ammonia gas. The process proposed in this Japanese Patent Publication comprises dipping an austenitic stainless steel article, for example a steel pipe, in a bath of a molten flux maintained at a temperature of from 1050.degree. to 1150.degree. C. to anneal it, the bath comprising in % by weight 48 to 65% of Na.sub.2 B.sub.4 O.sub.7, 35 to 43% of H.sub.3 BO.sub.3, 3 to 10% of NaH.sub.2 PO.sub.4 and 3 to 8% of NaF, taking the annealed article out of the bath to form coagulated flux coatings on the surfaces of the article, and quenching the article with warm water to destroy and peel off the coatings from the article. However, the coating of the coagulated flux having the prescribed composition is not very resistant to water and dissolves to some extent in the quenching water and in the washing water. Because of the presence of NaH.sub.2 PO.sub.4 in the flux and because of a recent severe requirement for control of P in waste water a suitable waste water disposal plant is required. Furthermore, non-oxidation of the steel is not completely ensured. This is partly because the protective property of the coagulated coatings (that is the ability of the coagulated coatings to protect the steel from being oxidized) is insufficient and partly because the article is quenched with water immediately after it has been taken out of the bath and, in consequence, the coagulated coatings are destroyed while the steel article still remains very hot. Accordingly, apart from the fact that the Japanese Patent Publication discloses batchwise annealing of a small article (e.g. a steel pipe of a length of 20 cm), an improvement in the flux composition is desired from the viewpoint of the large scale operation involved in continuous non-oxidative annealing of a continuously running steel strip, recovery and re-use of the used flux in such a large scale operation and treatment of waste water. Also, it is desirable to provide a process for annealing steel strip within a very short period of time. It is especially desirable to provide a continuous process for annealing cold rolled stainless steel strip substantially without forming oxide scales on the surfaces of the strip, particularly in an improved molten glass bath suitable for use in continuous non-oxidative annealing of cold rolled stainless steel strip.